Serum samples from 135 patients treated with human leukocyte alpha interferon (Hu IFN (Le) (New York Blood Center, Melville, N.Y. ) for Juvenile Laryngeal Papillomatosis (J.L.P.) were tested for the presence of neutralizing antibodies to the above mentioned alpha interferon. Eleven samples were found to contain neutralizing antibodies to alpha interferon from the N.Y.B.C. Ten of these contained neutralizing antibodies to Human leukocyte alpha interferon (Alferon; Interferon Sciences, New Brunswick, N.J.) and nine contained neutralizing antibodies to recombinant alpha interferon (Intron A, Schering-Plough, Kenilworth, N.J.). None of the samples were seen to bind to either Alferon or Intron in ELISA assays. Six, however, did bind to Wellferon (Burroughs Wellcome, Beckenham, U.K.) which is a natural lymphoblastoid interferon. Since we purify IFN-a components in our laboratory using Wellferon as starting material, we were interested to determine if there was any binding of the positive samples to our purified components. We found a low level of binding by three of the samples to an eluate which contains two of the species. Although the quantity of the aforementioned species is a limiting factor, we are in the process of running assays which will show if the antibody present reacts with one or both of these components.